1. Field of the Invention
The present invention relates to an active matrix type liquid crystal display device, and particularly to an active matrix type liquid crystal display device of an IPS (In-Plane Switching) system (=transverse electric field system).
2. Description of Related Art
An active matrix type liquid crystal display device using an active element such as a thin film transistor (TFT) is known. The active matrix type liquid crystal display device can increase pixel density, is small and lightweight, and consumes less power, so that as a substitute for a CRT, a product such as a monitor of a personal computer or a liquid crystal television has been developed. Especially, a technique of forming an active layer of a TFT by a crystalline semiconductor film typified by polycrystalline silicon makes it possible to form a driver circuit as well as a switching TFT for a pixel portion (hereinafter referred to as a pixel TFT) on the same substrate, and is ranked as a technique to contribute to miniaturization and weight lightening of a liquid crystal display device.
In the liquid crystal display device, a liquid crystal is sealed between a pair of substrates, and liquid crystal molecules are oriented by an electric field which is applied between a pixel electrode (individual electrode) of one of the substrates and an opposite electrode (common electrode) of the other substrate and is approximately vertical to a substrate plane. However, such a driving method of a liquid crystal has a defect that an angle of view is narrow, that is, although a normal display state is obtained when it is viewed in a direction vertical to the substrate plane, a color tone is changed and becomes unclear when it is viewed in an oblique direction.
As a method of overcoming this defect, there is an IPS system. This system has a feature that both a pixel electrode and a common wiring are formed on one of substrates and an electric field is changed to a transverse direction, and liquid crystal molecules do not rise but their orientation is controlled in the direction almost parallel with a substrate plane. By this operation principle, the angle of view can be widened.